1. Field of the Invention
The invention relates to real-time monitoring of molecular gas-solid surface, chemical, and physical interactions for purposes including detection of airborne molecular contaminants pertaining to manufacturing and processing environments.
2. Statement of the Problem
Many manufacturing processes and technologies are susceptible to airborne or gas-phase molecular contaminants (AMC), and to the related surface molecular contamination (SMC) resulting from chemical interactions between AMC and critical surfaces exposed to same. Such critical surfaces, called “subject surfaces” herein, are, for example: integrated circuit surfaces, such as resist, silicon and other semiconductors; wiring surfaces made of tungsten, aluminum, or other metals; silicon dioxide surfaces; optical surfaces; mechanical surfaces; surfaces of hard disks; surfaces of flat panel displays; etc. Detrimental effects of SMC include, for example, changes in the chemical, electrical, and optical qualities of critical surfaces. These detrimental effects decrease product performance and reliability, and raise product cost. Some examples of such detrimental effects to the above-mentioned critical surfaces include T-topping of resist, defective epitaxial growth, unintentional doping, uneven oxide growth, changes in wafer surface properties, corrosion and decreased metal pad adhesion. Many of these are becoming particularly detrimental as line widths less than 0.18 microns are being used. Further, as wafer size increases and device geometry decreases, the demand for more sensitive monitoring techniques will increase. In the optics industry, SMC is a well-known cause of hazing of optical surfaces. SMC also causes striction problems in certain mechanical devices, such as hard disk drives, since SMC contaminated surfaces may have a significantly higher coefficient of friction than uncontaminated surfaces. SMC also affects the manufacture of hard disk drives and flat panel displays which, for reasons known in the art, are typically carried out in a plurality of “mini” clean rooms.
The various AMCs causing detrimental SMC may be grouped into four general categories, which are: acids, bases, condensables, and dopants, otherwise referred to as SEMI F21-95 Classes A, B, C and D. Some AMCs, though, are of no particular class.
Sources for AMC include inadequate filtration of recirculated air, cross-process chemical contamination, outgassing of cleanroom materials, such as filters, gel sealants and construction materials, as well as contaminants carried in and exhuded by human beings. When the fluid is outdoor “make-up” air, the sources of AMC include automobile exhaust, evapotranspiration from plants, and various industrial emissions. The AMC also includes chemical compounds and vapors resulting from chemical breakdown of, and interaction between, the molecules within the AMC from the primary sources. Other sources of AMC/SMC include cross-process chemical contamination within a bay or across a facility, and recirculated air with inadequate ventilation. Still other sources include outgassing by cleanroom materials, such as filters, gel sealants, and construction materials, especially new fabrics, and various contaminants emanating from industrial equipment, such as pumps, motors, robots and containers. Another source is accidents, including chemical spills, and upsets in temperature and humidity controls. Still another source is people, including their bodies, clothes, and their personal care products.
AMC can cause yield losses even when present at concentrations as low as the low parts per billion by volume (“ppbv”). Such processes therefore require an ultra-clean, well-monitored environment.
No single monitoring instrument or technology is capable of monitoring across the four categories of AMC. Most existing technology instead focuses on subsets of the AMC categories, or particular species of AMC within one of the categories.
Another shortcoming of the existing monitoring technologies is that most monitor airborne, or ambient suspension, concentrations of AMC. Quartz crystal microbalances (QCM) and existing surface acoustic wave (SAW) technologies monitor SMC, but only on the surface of the QCM or SAW. Monitoring of accumulated SMC on the surface of the QCM crystal or SAW substrate may not provide accurate measurement of, or insight into, the chemical interactions at a critical surface. One significant reason for this failure is that the nature of the chemical interactions between the AMCs and the subject surface depends not only on the airborne or ambient concentration of the AMC, but on other factors such as the extent and nature of other AMC on the surface, temperature and relative humidity and, of notable importance, the chemical makeup of the critical surface. Therefore, when air samples are evaluated for AMC, the measured concentrations are converted to estimated quantities of molecular contamination on the subject surfaces. Typically, the estimation is based on sample duration or exposure time and the deposition rate:S=E(NV/4),  (1)where                S is the deposition rate in molecules/cm squared/sec;        E is a dimensionless sticking coefficient having a value ranging between 0 and 1;        N is the number density in air in molecules per cm cubed; and        V is the average thermal velocity in cm per second.        
E depends on temperature, humidity, surface composition, and the magnitude and type of other AMC present. E can only be derived experimentally and it is different for virtually every situation. Thus, it is recognized as a consistent and significant source of error.
The only commercially available approach that attempts to provide this data is the QCM. However, QCMs typically operate between 4 MHz and 12 MHz. SAWs typically operate at frequencies of 200 MHz and higher. Mass sensitivity of piezoelectric devices, both QCM and SAW, is proportional to the square of the frequency. QCM mass sensitivity is therefore, conservatively, two orders of magnitude poorer than that of a SAW mass. The lower mass sensitivity translates to poorer time resolution of mass change events, such as SMC accumulating on the QCM crystal as a result of an AMC event. Stated differently, if a SAW and a QCM device are exposed to the same AMC concentration, the SAW device has a rate of frequency change approximately two orders of magnitude higher than the QCM device. Therefore, the QCM must be exposed longer than the SAW to exhibit the same frequency change. This may effectively blur or miss contamination events that happen on short time scales. For this reason, QCM technology does not have sufficient mass sensitivity for adequate monitoring of critical surfaces exposed to short duration AMC. One such example is the monitoring of photoresists for exposure ammonia and amines, which may occur only over a two-hour period.
SAW devices have been used for detecting molecular contamination. See U.S. Pat. No. 6,122,954 issued Sep. 26, 2000 and U.S. Pat. No. 5,918,258 issued Jun. 29, 1999, both to William D. Bowers. In these devices, a fluid containing a contaminant is brought into contact with the SAW under conditions that promote deposition of the contaminant on the SAW, such as by evaporation of the fluid to leave the contaminants, or by covering the SAW with a hydroscopic polymer to enhance absorption of water, in cases where water is a contaminant. However, the first method is slow, and the second is limited both by types of contaminants that may be detected and reuseability of the SAW.